In recent years, progress has been made in research and development of diverse functional elements that use organic semiconductors. Organic EL elements are among the most common of such functional elements. An organic EL element is a current-driven light emitter that includes a pair of electrodes, consisting of an anode and a cathode, and a functional layer disposed between the pair of electrodes. The functional layer includes a light-emitting layer formed from organic material. The emission of light from the organic EL element is caused by an electric-field light-emitting phenomenon taking place as a result of the recombination of holes injected from the anode to the functional layer and electrons injected from the cathode to the functional layer after application of voltage between the electrodes. Given the high visibility of organic EL elements resulting from their self-luminescence, as well as their excellent vibration resistance resulting from the solid-state structure thereof, more attention is now being given to the application of organic EL elements as a light emitter for various display devices or as a light source.
In order for an organic EL element to emit high intensity light, efficient injection of carriers (holes and electrons) from the electrodes to the functional layer is essential. Generally, provision of an injection layer in between each of the electrodes and the functional layer is effective in realizing efficient injection of carriers to the functional layer, since an injection layer has the function of lowering the energy barrier during injection. An organic material, such as copper phthalocyanine or PEDOT (conducting polymer), or a metal oxide, such as molybdenum oxide or tungsten oxide, is used as the hole injection layer provided between the functional layer and the anode. An organic material, such as a metal complex or oxadiazole, or a metal such as barium is used as the electron injection layer provided between the functional layer and the cathode.
Among such injection layers, an improvement in hole conduction efficiency as well as longevity of the organic EL element has been reported for an organic EL element using a metal oxide film composed of a metal oxide, such as molybdenum oxide or tungsten oxide, as the hole injection layer (see Patent Literature 1, 2, and Non-Patent Literature 1).